A particulate matter detection system is known, which is equipped with a particulate matter (PM) detection sensor, a current detection part connected to the particulate matter detection sensor and a control circuit connected to the particulate matter detection sensor and the current detection part. For example, Patent document 1 discloses a conventional particulate matter detection system. The particulate matter detection sensor has a pair of electrodes and a heater for heating the electrodes.
The control circuit is configured to perform a switching control of a detection mode and a burning mode. In the detection mode, a voltage is supplied between the pair of electrodes in the particulate matter detection sensor. In the detection mode, particulate matter is collected and accumulated between the pair of electrodes by electrostatic force and a current flows between the pair of electrodes due to the accumulation of particulate matter. The current detection part detects the current, and the control circuit part calculates an amount of particulate matter on the basis of the detected current.
Particulate matter is collected between the electrodes after long execution of the detection mode. As a result, the current flowing between the electrodes becomes saturated. After the saturation of the current flowing between the electrodes, the control circuit switches to the burning mode from the detection mode, and burns the accumulated particulate matter so as to remove the accumulated particulate matter from the particulate matter detection sensor. This regenerates the particulate matter detection sensor.
The current detection part is composed of a plurality of electronic components such as operational amplifiers, etc. Each of the electronic components varies electrical characteristics thereof due to a temperature, etc. For this reason, there is in general a difference (i.e. an offset value) between a current value detected by the current detection part and a correct current value which is proportional to an amount of particulate matter accumulated between the electrodes. Accordingly, an incorrect amount of particulate matter contained in exhaust gas is often calculated due to only using the current value detected by the current detection part.
In order to avoid this problem and to accurately detect an amount of particulate matter contained in exhaust gas, there has been proposed a method of correcting the current value detected by the current detection part, and of calculating an accurate current value flowing between the pair of electrodes on the basis of the corrected current value. That is, after the burning mode and before starting of the detection mode, the current detection part detects a current flowing between the pair of electrodes, and stores the detected current value as an offset value into a memory. When particulate matter accumulated between the pair of electrodes has been burned completely, no current flows between the pair of electrodes. Accordingly, it is possible to consider that the detected current value obtained by the current detection part is substantially equal to the offset value. It is therefore possible to detect a true current value flowing between the pair of electrodes and to accurately detect an amount of particulate matter contained in exhaust gas by subtracting the offset value from the detected current value obtained in the detection mode.